Radioactive isotopes provide a time scale for oceanographic studies. The ideal isotopic tracer is an integral part of the water, has a convenient half-life, and is easily measurable. No tracer is ideal for all oceanographic studies. Unless great care is taken in the interpretation, the measurements may tell us that the particular isotope is not suitable for the problem selected rather than that the ocean is behaving strangely.

There are three oceanic regions where isotopic tracers are especially useful as mixing indicators: (1) the thermocline; (2) the ocean floor; (3) intermediate currents. Different isotopes are useful in each area; however, confusion may arise if the geochemical behaviour of the isotope in the ocean is not well understood.

Mixing through the main thermocline is best studied using cosmic ray-produced isotopes (C¹⁴, H³, Si³⁸) and hydrogen bomb-produced isotopes (Sr⁹⁰, Cs¹⁹⁷, H³, C¹⁴) The long half-life (5700 years) and geochemical reactions of C" make results based on it alone rather uncertain. The other isotopes tell us that mixing through the thermocline is an extremely slow process characterised by a diffusion coefficient of lcm=/sec. Mixing near the ocean floor is studied using isotopes which naturally leak from deep sea sediments. Koczy attempted to use the diffusion of radium-226 as a mixing indicator; however, the long half-life (1600 years) and geochemical uncertainties have rendered Ra'" almost useless. Broecker found that the Ra'",' daughter, radon-222, does not suffer these difficulties. It is a rare gas with a 3.8 day half-life and is a very convenient indicator of mixing in the lower 100 meters of the water column. Other isotopes that have potential in such studies are radium-228 (half-life = 6.7 years) and lead-210 (half-life = 22 years). Both are undergoing extensive geochemical tests.

Intermediate currents whose waters originate near the surface may also be studied using bomb-produced and cosmic ray-produced isotopes. The outflows from the Red Sea, the Persian Gulf, and the Mediterranean Sea are prime examples of currents that lend themselves to such studies. No one has undertaken this work to date in spite of the rich opportunity for significant contributions.